Reversible phase change composition for storing energy

ABSTRACT

A reversible liquid/solid phase change composition for the storage of energy comprising a mixture of hydrated calcium bromide/calcium chloride and a modifier selected from KBr, KCl, or mixtures thereof, for modifying the semicongruent melting of the mixture to reduce the formation of crystalline CaBr 2  and CaCl 2  hydrate phases other than the hexahydrate phase. The composition is useful over a temperature range of from about 5° to about 50° C. and can be employed for the storage of coolness or heat, or as a buffer or heat sink to modulate diurnal swings in ambient temperature.

RELATED APPLICATION

This application is a continuation-in-part application of our copendingapplication Ser. No. 504,763, filed June 15, 1983, now abandoned.

BACKGROUND OF THE INVENTION

The invention relates to reversible liquid/solid phase changecompositions (PCM). More particularly, the invention resides in phasechange compositions for the storage of energy comprising a mixture ofhydrated calcium bromide and hydrated calcium chloride having addedthereto a modifier selected from the group consisting of KBr, KCl, andmixtures thereof.

The modifier which is employed with the hydrated CaBr₂ /CaCl₂ mixture ofthe invention is particularly effective to modify the semicongruentmelting behavior of the mixture to reduce the formation of crystallineCaBr₂ and CaCl₂ hydrate phases other than the hexahydrate phase.

Preferably, the PCM of the invention also includes a nucleating agentwhich is particularly effective to reduce supercooling of the PCM toless than 3° C. during retrieval of the stored energy bycrystallization. The nucleating agents which have been found to beeffective in the quaternary or quinary PCM's of the invention areselected from the group consisting of SrCl₂, Ba(OH)₂, BaO, SrBr₂,Sr(OH)₂, SrI₂, BaI, BaCO₃, SrCO₃, and mixtures thereof.

The invention also resides in the use of the PCM of the particularcomposition as herein described in an energy stored device and in themethod of storing energy.

The phase change compositions of the invention are useful over a broadtemperature range of from about 5° to about 50° C. and thus can beemployed (1) for the storage of "coolness" over a temperature range offrom about 5° to about 17° C.; (2) for the storage of "heat" over atemperature range of from about 25° to about 50° C., and (3) as a bufferor heat sink to modulate diurnal swings in the ambient temperature rangeof from about 17° C. to about 27° C.

DESCRIPTION OF THE PRIOR ART

Phase change materials in which the heat of fusion of various hydratedsalt compositions is employed are well known in the literature. InASHRAE Journal of September 1974, pages 38-45, M. Telkes in an articleentitled "Solar Energy Storage", evaluated the thermal, physical andother pertinent properties of various PCM's on the basis of economics,applicability, corrosion, toxicity, and availability for large scaleinstallations.

A more comprehensive discussion of PCM's for use as thermal energystorage materials is given in Solar Heat Storage, Latent Heat Materials,by G. A. Lane, CRC Press, Boca Raton, Fla., 1983, pages 2-48, 143-148.

Particular reference is also made to the following prior publications:

(1) Japanese Publication No. 81/08483, Mitsubishi Electric Corporation,published Jan. 28, 1981, which discloses a PCM comprising CaCl₂.6H₂ Oand/or CaBr₂ ·6H₂ O which is mixed with a small amount of an agent forpreventing supercooling such as BaHPO₄ and other barium salts.

(2) Japanese Pat. No. 56-141380, Mitsubishi Electric Corporation,discloses a CaBr₂ /CaCl₂ PCM hydrated with 6.2 to 8.4 moles of H₂ O,where the CaCl₂ is present in an amount of from 50 to 90 mole percentand the CaBr₂ is present in an amount of from 10 to 50 mole percent ofthe salt. This does not teach the addition of a crystal phase modifierof KCl and/or KBr, nor any of the select nucleators of the invention.

(3) Japanese Pat. No. J57-139168, published Aug. 27, 1982, teaches theaddition of KBr to minimize supercooling of CaCl₂.6H₂ O. The patent doesnot disclose mixtures of hydrated CaCl₂ /CaBr₂ PCM's nor does it teachthe addition of a crystal phase modifier such as KCl and/or KBr or anyof the select nucleators of the present invention.

(4) Japanese Pat. No. J51-070193, published June 17, 1976, discloses ahydrated CaBr₂ or CaCl₂ PCM. It does not disclose a mixture of hydratedCaBr₂ /CaCl₂. The patent does not employ the Crystal Phase Modifier ofKCl and/or KBr nor the select nucleators employed in the invention ofthe subject application.

(5) Japanese Pat. No. J53-019183, published Feb. 22, 1978, discloses theaddition of SrCl₂ or SrBr₂ as nucleators to CaCl₂ ·6H₂ O. The patentdoes not disclose a phase change material comprising a mixture of ahydrated CaBr₂ /CaCl₂ nor the addition of a Crystal Phase Modifier ofKCl and/or KBr to the basic PCM.

(6) Japanese Pat. No. J57-180684, published Nov. 6, 1982, discloses ahydrated CaBr₂ PCM. The patent does not disclose a mixture of a hydratedCaBr₂ /CaCl₂ composition nor does it disclose the addition of a CrystalPhase Modifier of KCl and/or KBr to the composition.

(7) Japanese Pat. No. 57-096079, published June 1982, discloses amixture of a hydrated CaBr₂ /CaCl₂ phase change composition. The patentdoes not provide for the addition of the Crystal Phase Modifier of KCland/or KBr as is required in the present invention.

(8) U.S. Pat. No. 4,392,971 (Kimura et al) discloses a heat storagematerial of CaCl₂ ·6H₂ O which has been modified by controlling thewater content to a molar ratio of greater than 6.0 and less than 6.14based on the CaCl₂ present in the heat storage material to prevent thecrystallization of CaCl₂.4H₂ O. Kimura also includes KCl, among others,as a nucleation promoting agent which is described as different fromordinary nucleating agents as it does not initiate nucleation itself.

In accordance with their teachings, Kimura, et al. reduce thethermodynamic tendency of the CaCl₂.6H₂ O to form the tetrahydrate byabandoning the hexahydrate stoichiometry of their composition by theadding of water in excess of 6.0 moles per mole of CaCl₂. In using extrawater, Kimura et al have employed phase equilibrium principles to reducethe formation of CaCl₂.4H₂ O without altering the phase diagram. Underthese conditions, however, deterioration can occur over repeated cyclesof freezing and thawing of the PCM due to a settling-out of the hydratecrystals with a consequent loss of heat storage capacity.

The present invention effectively reduces the formation of tetrahydratecrystals by altering the phase diagram, i.e. the phase equilibriumbehavior of the ternary CaCl₂ /CaBr₂ /H₂ O system so that thethermodynamic tendency to form unwanted tetrahydrate crystals issubstantially mitigated or totally eliminated. This is accomplished inaccordance with the teachings of the present invention by converting theternary system to a quaternary or quinary system of CaCl₂ /CaBr₂ /KCland/or KBr and H₂ O. With the addition of a sufficient amount of themodifiers KCl and/or KBr to the basic ternary mixture, substantiallyless or none of the tetrahydrate crystals can form in the PCM inaccordance with the laws of thermodynamics.

The nucleation promoting agents i.e. NaCl, KCl, etc, described byKimura, et al. are said to have excellent anti-supercooling effects.Kimura, et al. thus do not regard to recognize that their nucleationpromoting agents act to modify the phase equilibrium behavior, i.e. thesemicongruent melting behavior of their hydrated CaCl₂ composition. Thequaternary and quinary congruently melting systems of the presentinvention effectively prevent the formation of CaCl₂.4H₂ O over longperiods of time and over many cycles of freezing and thawing. No claimis made herein to heat storage materials as disclosed by Kimura, et al.in which a mixture of CaCl₂ /CaBr₂ /KCl is provided having a watercontent in a molar ratio of greater than 6.0 and less than 6.14 based onCaCl₂ and CaBr₂ present in the composition.

Publications (1) through (8) generally disclose PCM's of CaCl₂.6H₂ O ormixtures of CaBr₂.6H₂ O and CaCl₂.6H₂ O having various nucleators addedto the basic PCM. The present invention provides a hydrated CaCl₂ andCaBr₂ mixture having, in addition, a crystal phase modifier selectedfrom KCl, KBr, or mixtures thereof to modify the semicongruent meltingbehavior of the hydrated CaBr₂ /CaCl₂ mixture to reduce the formation ofcrystalline hydrate phases other than the CaBr₂ /CaCl₂.6H₂ O phase.

The present invention also provides for a PCM of CaCl₂ /CaBr₂ /KCl inwhich the hydration of the mixture is from about 28 weight percent waterup to about 6 mole of water per mole of calcium salt, and from greaterthan 6.14 moles of water per mole of calcium salt up to about 50 weightpercent water.

The PCM's of the invention are useful for the storage of "coolness" overa temperature range of from about 5° to about 17° C.; for the storage ofheat over a temperature range of from about 25° to about 50° C., and as"Buffers" or heat sinks for modulating diurnal fluctuations in theambient temperature range of from about 17° to about 27° C. PCM's whichact as buffers or heat sinks are particularly useful in passive solarheated buildings or structures such as greenhouses or hothouses. Excesssolar energy received during the daytime is used to melt or partiallymelt the PCM. At night, this energy is recovered by freezing the PCM andis transported or allowed to radiate to the building's other temperedspace to raise the temperature, i.e. to heat the building or space.

SUMMARY OF THE INVENTION

The present invention particularly resides in a reversible liquid/solidphase change composition having a melting temperature of from about 5°to about 50° C., said composition comprising a hydrated mixture of fromabout 20 to about 67 weight percent CaBr₂, from greater than zero toabout 38 weight percent CaCl₂, from about 28 to about 50 weight percentwater, and a modifier selected from the group consisting of KBr, andmixtures of KBr and KCl, said modifier being present in an amountgreater than zero to less than about 10 weight percent and sufficient tomodify the semicongruent melting behavior of the CaBr₂ /CaCl₂ mixture toreduce, during freezing of the composition, the formation of crystallineCaBr₂ and CaCl₂ hydrate phases other than the hexahydrate phase.

The invention also resides in a reversible liquid/solid phase changecomposition having a melting temperature of from about 5° C. to about50° C., said composition comprising a hydrated mixture of from about 20to about 67 weight percent CaBr₂, from greater than zero to about 38weight percent CaCl₂, from about 28 weight percent water up to about 6.0moles of water per mole of calcium salt and from greater than 6.14 molesof water per moles of calcium salt up to about 50 weight percent water,and from greater than zero to less than about 10 weight percent KCl butsufficient to modify the semicongruent melting behavior of the CaBr₂/CaCl₂ mixture to reduce, during freezing of the composition, theformation of crystalline CaBr₂ and CaCl₂ hydrate phases other than thehexahydrate phase.

The invention further resides in a method of storing energy, comprisingthe steps of preparing a reversible liquid/solid phase changecomposition which melts at a temperature from about 5° to about 50° C.,by admixing from about 20 to about 67 weight percent CaBr₂, from greaterthan zero to about 38 weight percent CaCl₂, from about 28 to about 50weight percent water, and a modifier in an amount greater than zero butless than about 10 weight percent and in an amount sufficient to modifythe semicongruent melting behavior of the CaBr/CaCl₂ mixture to reduce,during freezing of the composition, the formation of crystalline hydratephases other than the hexahydrate phase, said modifier being selectedfrom the group consisting of KBr, and mixtures of KBr and KCl,introducing the composition into an encapsulating means for use as anenergy storage device, and hermetically sealing the encapsulating meansto prevent the escape of water vapors from the encapsulating means.

The invention further resides in an energy storage device comprising anencapsulating means containing the reversible liquid/solid phase changecomposition of the invention, and sealing means for the encapsulatingmeans for hermetically sealing the composition in the encapsulatingmeans to prevent the evaporation of water from the composition.

The composition of the present invention preferably includes one or morenucleating agents in an amount sufficient to reduce supercooling of thecomposition to less than about 3° C. during crystallization. Preferably,the nucleating agent is present in an amount greater than zero to about5.0 weight percent. More preferably, the nucleating agent is present inan amount of from about 0.50 to about 2.0 weight percent. Althoughamounts greater than 5.0 weight percent may be present in thecomposition without detriment to the function of the composition,nucleators in amounts greater than 5.0 weight percent generally do notshow any further benefits in reducing supercooling.

DEFINITIONS

The term "enthalpy" used herein defines a thermodynamic function of asystem, equivalent to the internal energy plus the product of thepressure and the volume. Enthalpy is measured by the heat content perunit mass, e.g., in BTU's per pound.

The term "congruent melting" herein used defines a mixture ofingredients, based on hydrated CaBr₂ /CaCl₂ for which, at the meltingpoint, solid and liquid phases are in stable equilibrium, i.e., thesolid phase contains no hydrated CaBr₂ or CaCl₂ other than thehexahydrate or solid solutions thereof, and the liquid phase contains,for every mole of CaBr₂ and CaCl₂, 6 moles of water plus sufficientwater to form the stable hydrate of any additive materials in solution.

"Semicongruent melting" occurs when a phase change material has two ormore hydrate forms with differing solid compositions and melting points.the material can be transformed in other hydrate forms before eithercomplete melting or freezing occurs, resulting in a broadened meltingpoint range. In addition, there is the temporary loss in thermal storagecapacity. Calcium chloride hexahydrate is an example of asemicongruently melting phase change material.

"Incongruent melting" phase change materials yield two distinct phasesupon melting, i.e., a saturated solution and a precipitate of aninsoluble anhydrous salt. If the precipitate settles out of thesolution, the anhydrous salt will not hydrate completely upon coolingand some thermal storage capacity will be lost with each freeze/meltingcycle. Incongruent melting, as observed with sodium sulfate decahydrate,for example, is a more serious problem because it can result in acontinual loss of latent heat storage capacity.

The term "supercooling" refers to a discrepancy between the temperatureat which freezing initiates and the melting temperature of a givenliquid/solid phase change material when cooled and heated.

The term "eutectic" or "eutectic mixture" designates a mixture of two ormore components mixed in such a ratio that the melting point of themixture is lower than that of either salt, and the entire mixture at oneand the same temperature passes from the solid form into the liquidform, and vice versa.

The term "modifier" includes, in addition to the KCl, KBr, and mixturesthereof such as have been specified herein, the precursors of suchmodifiers which are non-detrimental to the function of the PCM's of theinvention. More particularly, the modifiers herein referred to areeither anhydrous or hydrated compositions of potassium salt precursorswhich would form the potassium salt upon addition to the hydrated CaBr₂/CaCl₂ mixture.

Impurities may also be present in the PCM of the invention in minoramounts of less than about 3.0 weight percent and provided that suchimpurities do not detrimentally affect the function of the basichydrated CaBr₂ /CaCl₂ mixture. Impurities may include, for example, LiClor other calcium salts such as CaCO₃ or CaSO₄.

DETAILED DESCRIPTION OF THE INVENTION

For air conditioning, a PCM should preferably melt at a temperature atleast 5° C. below the temperature of a room or space so that efficientheat exchange can be obtained between room air and the coolness storagematerial. Accordingly, at a room temperature of about 22° C. the maximumuseful PCM melting point is about 17° C. As the freezing point of thePCM approaches 0° C., the refrigeration equipment which is used tocharge the coolness storage increases in size and rating. Accordingly, aminimum practical PCM freezing point is about 5° C.

One aspect of the present invention resides in the development of a PCMfor storing "coolness." It has been discovered that a PCM based on aminimum-melting mixture of CaBr₂.6H₂ O and CaCl₂.6H₂ O melts at atemperature of about 16° C. This mixture is not fully congruent melting,however, since some tetrahydrate crystals can form during freezing,thereby decreasing the storage capacity of the PCM. This defect wasovercome by adding a sufficient amount of KCl, KBr, or mixtures thereofto the composition to modify the semicongruent melting behavior of themixture substantially congruently melting.

Examples of PCM's which are particularly well adapted for storage of"coolness" and melting in the temperature range of from 5° to 17° C. arethe following, in their order of preference with Example 3A being themost preferred composition:

EXAMPLE 1A

CaBr₂ --from about 28 to about 43 wt. %;

CaCl₂ --from about 14 to about 31 wt. %;

KBr or mixtures of KBr and KCl--in an amount of greater than zero andless than 10 wt. %;

H₂ O--from about 34 to about 48 wt. %.

EXAMPLE 2A

CaBr₂ --from about 30 to about 41 wt. %;

CaCl₂ --from about 18 to about 26 wt. %;

KBr or mixtures of KBr and KCl--from about 2 to about 5 wt. %;

H₂ O--balance of up to 100 wt. %.

EXAMPLE 3A

CaBr₂ --from about 32 to about 37 wt. %;

CaCl₂ --from about 20 to about 24 wt. %;

KBr or mixtures of KBr and KCl--from about 3 to about 4 wt. %;

H₂ O--balance of up to 100 wt. %.

EXAMPLE 4A

CaBr₂ --from about 28 to about 43 wt. %;

CaCl--from about 14 to about 31 wt. %;

KCl--in an amount greater than zero and less than 10 wt. %;

H₂ O--balance up to 100 wt. % but excluding an amount of greater than6.0 and less than 6.14 moles of water per mole of CaCl₂ plus CaBr₂.

Examples of PCM's which are particularly well adapted as heat storagematerials melting in the range of from about 25° up to about 50° C. arethe following, in their order of preference with Example 3B being themost preferred composition:

EXAMPLE 1B

CaBr₂ --from about 47 to about 67 wt. %;

CaCl₂ --greater than zero but less than about 10 wt. %;

KBr or mixtures of KBr and KCl--in an amount greater than zero and lessthan about 10 wt. %;

H₂ O--from about 29 to about 43 wt. %.

EXAMPLE 2B

CaBr₂ --from about 50 to about 66 wt. %;

CaCl₂ --greater than zero but less than about 10 wt. %;

KBr or mixtures of KBr and KCl--from about 2 to about 5 wt. %;

H₂ O--from about 31 to about 40 wt. %.

EXAMPLE 3B

CaBr₂ --from about 52 to about 65 wt. %;

CaCl₂ --greater than zero but less than about 10 wt. %;

KBr or mixtures of KBr and KCl--from about 3 to about 4 wt. %;

H₂ O--from about 33 to about 36 wt. %.

EXAMPLE 4B

CaBr₂ --from about 47 to about 67 wt. %;

CaCl₂ --greater than zero but less than about 10 wt. %;

KCl--in an amount greater than zero and less than about 10 wt. %;

H₂ O--from about 28 wt. % water up to about 6.0 moles of water per moleof calcium salt and from greater than 6.14 moles of water per mole ofcalcium salt up to about 50 wt. % water.

Examples of PCM's which are particularly well adapted as temperaturebuffers or heat sinks in passive solar energy heated structures meltingin the temperature range of from about 17° to about 27° C. are thefollowing, in their order of preference with Example 3C being the mostpreferred composition:

EXAMPLE 1C

CaBr₂ --from about 38 to about 55 wt. %;

CaCl₂ --from about 10 to about 25 wt. %;

KBr or mixtures of KBr and KCl--in an amount greater than zero and lessthan about 10 wt. %;

H₂ O--from about 29 to about 43 wt. %;

EXAMPLE 2C

CaBr₂ --from about 42 to about 55 wt. %;

CaCl₂ --from about 10 to about 17 wt. %;

KBr or mixtures of KBr and KCl--from about 2 to about 5 wt. %;

H₂ O--from about 36 to about 40 wt. %.

EXAMPLE 3C

CaBr₂ --from about 46 to about 52 wt. %;

CaCl₂ --from about 10 to about 15 wt. %;

KBr or mixtures of KBr and KCl--from about 2 to about 5 wt. %;

H₂ O--from about 37 to about 39 wt. %.

EXAMPLE 4C

CaBr₂ --from about 38 to about 55 wt. %;

CaCl₂ --from about 10 to about 25 wt. %;

KCl--in an amount greater than zero and less than about 10 wt. %;

H₂ O--from about 28 wt. % water up to about 6.0 moles of water per moleof calcium salt and from greater than 6.14 moles of water per mole ofcalcium salt up to about 50 wt. % water.

Examples of PCM's which are particularly well adapted as temperaturebuffers or heat sinks in passive solar energy heated structures meltingin the temperature range of from about 17° to about 27° C. are thefollowing, in their order of preference with Example 3D being the mostpreferred composition:

EXAMPLE 1D

CaBr₂ --from about 20 to about 28 wt. %;

CaCl₂ --from about 21 to about 28 wt. %;

KBr or mixtures of KBr and KCl--in an amount greater than zero and lessthan about 10 wt. %;

H₂ O--from about 33 to about 50 wt. %.

EXAMPLE 2D

CaBr₂ --from about 20 to about 28 wt. %;

CaCl₂ --from about 25 to about 36 wt. %;

KBr or mixtures of KBr and KCl--from about 2 to about 5 wt. %;

H₂ O--from about 40 to about 48 wt. %.

EXAMPLE 3D

CaBr₂ --from about 20 to about 28 wt. %;

CaCl₂ --from about 28 to about 35 wt. %;

KBr or mixtures of KBr and KCl--from about 3 to about 4 wt. %;

H₂ O--from about 43 to about 46 wt. %.

EXAMPLE 4D

CaBr₂ --from about 20 to about 28 wt. %;

CaCl₂ --from about 21 to about 38 wt. %;

KCl--in an amount greater than zero and less than about 10 wt. %;

H₂ O--from about 28 wt. % water up to about 6.0 moles of water per moleof calcium salt and from greater than 6.14 moles of water per mole ofcalcium salt up to about 50 wt. %.

Although the amount of KBr and/or KCl present in each of the abovecompositions is stated to be greater than zero weight percent it shouldbe sufficient to improve the freezing behavior of the PCM. It has beenfound that an amount of greater than 10 weight percent generally has noadditional benefit in the freezing behavior of the PCM.

The avoidance of supercooling during the crystallization of hydratedPCM's, as by the addition of various nucleating agents, is generallyknown in the literature.

The present invention preferably also provides for the addition of theselect nucleating agents herein before enumerated in order to reducesupercooling in the hydrated CaBr₂ /CaCl₂ /KCl and/or KBr mixtures.

The following examples illustrate the effectiveness of reversibleliquid/solid phase change compositions of the invention for storage ofenergy based on mixtures of hydrated CaBr₂ and CaCl₂.

EXAMPLE 5

This example is not an example of the invention but is included to showhow a minimum-melting mixture of CaBr₂.6H₂ O and CaCl₂.6H₂ O wasidentified. A container of melted CaBr₂.6H₂ O was cooled until asubstantial quantity of crystals had been formed and an equilibrium hadbeen established. Melted CaCl₂.6H₂ O was then added by increments,allowing equilibrium to be established after each addition. At theequilibrium condition, the equilibrium temperature of the liquidcomposition was determined. The liquid composition was determined bychemical and instrument analysis. The minimum-melting composition wasfound at a temperature of 15.8° C. and contained 54.0 weight percentCaBr₂.6H₂ O and 46.0 weight percent CaCl₂.6H₂ O.

EXAMPLE 6

This example is also not an example of the invention but shows that theminimum-melting composition of Example 5 is semicongruently melting,that is, at equilibrium the tetrahydrate species crystallize initiallybefore hexahydrate crystals begin to form. A container of the followingmelted salt mixture comprising 32.0 weight percent CaBr₂ ; 21.0 weightpercent CaCl₂, and 47.0 weight percent H₂ O was prepared and cooleduntil a substantial quantity of crystals had formed and equilibrium hadbeen established. A melted material of the composition comprising 38.0weight percent CaBr₂ ; 25.3 weight percent CaCl₂, and 36.7 weightpercent H₂ O, was then added incrementally, allowing equilibrium to beattained after each addition, and determining equilibrium temperatureand liquid composition. From the data thus generated, the hexahydrateand tetrahydrate liquidi were obtained, and their intersection point,the peritectic point, was found to be at 16.3° C. and the compositionwas analyzed at 34.0 weight percent CaBr₂ ; 23.2 weight percent CaCl₂,and 42.8 weight percent H₂ O. This composition was analyzed to contain6.27 moles of water per gram atom of calcium. At a ratio of 6.00 molesof water per gram atom of calcium, the tetrahydrate liquidus was foundto be at a temperature of 19.3° C. Thus, the hexahydrate compositionmust cool about 3.0° C., forming tetrahydrate crystals, at equilibrium,before the hexahydrate crystals can begin to form. This segregation ofthe material causes reduced heat storage capacity for the PCM.

EXAMPLE 7

This experiment is an example of the invention and was conducted in thesame manner as Example 5, except that the starting material wasCaBr₂.6H₂ O saturated with KBr, and the material added incrementally wasCaCl₂.6H₂ O saturated with KCl. The minimum melting composition wasfound at a temperature of 14.6° C. and the composition contained 53.9weight percent CaBr₂.6H₂ O; 42.9 weight percent CaCl₂.6H₂ O; 1.9 percentKBr, and 1.3 percent KCl.

EXAMPLE 8

This experiment is an example of the invention and was conducted in thesame manner as Example 6, except that the starting material was of thefollowing composition: 32.9 weight percent CaBr₂ ; 19.7 weight percentCaCl₂ ; 41.4 weight percent H₂ O; 3.0 weight percent KBr, and 3.0 weightpercent KCl. A mixture of the following composition was addedincrementally: 37.5 weight percent CaBr₂ ; 22.5 weight percent CaCl₂ ;33.9 weight percent H₂ O; 3.0 weight percent KBr, and 3.0 weight KCl. Inthis example, however, the hexahydrate and tetrahydrate liquidi werefound to intersect at a eutectic (rather than peritectic) point at atemperature of 14.7° C. The ratio of water to calcium was about 5.9moles of water per gram atom of calcium. Further, at a ratio of 6.00moles of water per gram atom of calcium, the hexahydrate liquidus wasfound to be at a temperature of 14.7° C., indicating a congruent-meltingsystem with no tetrahydrate formation at equilibrium.

EXAMPLE 9

This experiment is an example of the invention and was conducted in thesame manner as Example 6, except that the starting material was of thefollowing composition: 43.4 percent CaBr₂ ; 13.3 percent CaCl₂ ; 39.7percent H₂ O; 2.7 percent KBr, 0.9 percent KCl; and the material whichwas added incrementally having the following composition: 46.0 percentCaBr₂, 13.6 percent CaCl₂ ; 36.7 percent H₂ O; 2.7 percent KBr, 0.9percent KCl. The hexahydrate and tetrahydrate liquidi were found tointersect at a temperature of 19.7° C., and at about 6.09 moles of waterper gram atom of calcium, indicating a system in which very littletetrahydrate can form.

EXAMPLE 10

This experiment is an example of the invention and was conducted in thesame manner as Example 6, except that the starting material was of thefollowing composition: 48.0 weight percent CaBr₂ ; 9.6 weight percentCaCl₂ ; 38.4 weight percent H₂ O; 3.3 weight percent KBr, and 0.8 weightpercent KCl; and the material added incrementally had the composition:51.1 weight percent CaBr₂, 10.2 weight percent CaCl₂ ; 35.1 weightpercent H₂ O; 2.9 weight percent KBr, and 0.7 weight percent KCl. Thehexahydrate and tetrahydrate liquidi were found to intersect at atemperature of 24° C., and at about 5.8 moles of water per gram atom ofcalcium, indicating a system in which no tetrahydrate will form atequilibrium.

EXAMPLE 11

This experiment is an example of the invention in which a samplecontaining 34.57 weight percent CaBr₂ ; 21.98 weight percent CaCl₂ ;40.19 weight percent H₂ O; 1.90 weight percent KBr, and 1.36 weightpercent KCl was divided into several aliquots, and freezing-meltingtests were conducted. One of the aliquots was tested without anyadditives, but various nucleating agents were added to the otheraliquots at the level of 0.5 weight percent in order to determine theeffectiveness of such nucleators in the mixture. In each case, tenfreeze-thaw cycles were induced by immersing a glass container with 80grams of the material alternately in ice water and room temperature.Temperature of the sample was obtained from a thermocouple placed in thecenter of the sample. For each freezing cycle, the amount ofsupercooling (cooling below the melting point before onset ofcrystallization) was noted and these values were averaged for the tencycles. Table I summarizes the data obtained with the addition ofselected nucleators.

                  TABLE I    ______________________________________    Effect of Nucleators on Supercooling    Nucleators  Melting Point, °C.                             Supercooling, °C.    ______________________________________    None        13-15        3.4    SrCl.sub.2  13-15        1.0    Ba(OH).sub.2                12-15        1.1    BaO         12-15        1.2    SrBr.sub.2  13-16        1.4    Sr(OH).sub.2                13-15        2.1    SrI.sub.2   12-15        2.6    BaI.sub.2     13-14.5    2.8    BaCO.sub.3  14-15        2.9    ______________________________________

All of the nucleating additives listed in Table I were found to beeffective in reducing supercooling of the PCM of the present invention.

What is claimed is:
 1. A reversible liquid/solid phase changecomposition having a melting temperature of from about 5° to about 50°C., said composition comprising a hydrated mixture of from about 20 toabout 67 weight percent CaBr₂, from greater than zero to about 38 weightpercent CaCl₂, from about 28 to about 50 weight percent water, and amodifier selected from the group consisting of KBr, and mixtures of KBrand KCl, said modifier being present in an amount greater than zero toless than about 10 weight percent and sufficient to modify thesemicongruent melting behavior of the CaBr₂ /CaCl₂ mixture to reduce,during freezing of the composition, the formation of crystalline CaBr₂and CaCl₂ hydrate phases other than the hexahydrate phase.
 2. Thecomposition of claim 1, wherein said mixture comprises from about 28 toabout 43 weight percent CaBr₂, from about 14 to about 31 weight percentCaCl₂, and from about 34 to about 48 weight percent water.
 3. Thecomposition of claim 2, wherein said mixture comprises from about 30 toabout 41 weight percent CaBr₂, from about 18 to about 26 weight percentCaCl₂, from about 2 to about 5 weight percent of the modifier, with theremainder being water in an amount of up to 100 weight percent of thecomposition.
 4. The composition of claim 2, wherein said admixturecomprises from about 32 to about 37 weight percent CaBr₂ ; from about 20to about 24 weight percent CaCl₂, from about 3 to about 4 weight percentof the modifier, with the remainder being water in an amount of up to100 weight percent of the composition.
 5. The composition of claim 1,wherein said mixture comprises from about 38 to about 55 weight percentCaBr₂, from about 10 to about 25 weight percent CaCl₂, and from about 29to about 43 weight percent water.
 6. The composition of claim 5, whereinsaid mixture comprises from about 42 to about 55 weight percent CaBr₂,from about 10 to about 17 weight percent CaCl₂, and from about 36 toabout 40 weight percent water, said modifier being present in an amountof from about 2 to about 5 weight percent.
 7. The composition of claim5, wherein said mixture comprises from about 46 to about 52 weightpercent CaBr₂, from greater than 10 to about 15 weight percent CaCl₂,and from about 37 to about 39 weight percent water, said modifier beingpresent in an amount of from about 3 to about 4 weight percent.
 8. Thecomposition of claim 1, wherein said mixture comprises from about 20 toabout 28 weight percent CaBr₂, from about 21 to about 38 weight percentCaCl₂, and from about 33 to about 50 weight percent water.
 9. Thecomposition of claim 8, wherein said mixture comprises from about 20 toabout 28 weight percent CaBr₂, from about 25 to about 36 weight percentCaCl₂, and from about 40 to about 48 weight percent water, said modifierbeing present in an amount of from about 2 to about 5 weight percent.10. The composition of claim 8, wherein said mixture comprises fromabout 20 to about 28 weight percent CaBr₂, from about 28 to about 35weight percent CaCl₂, and from about 43 to about 46 weight percentwater, said modifier being present in an amount of from about 3 to about4 weight percent.
 11. The composition of claim 1, wherein said mixturecomprises from about 47 to about 67 weight percent CaBr₂, from greaterthan zero to less than about 10 weight percent CaCl₂, and from about 29to about 43 weight percent water.
 12. The composition of claim 11,wherein said mixture comprises from about 50 to about 66 weight percentCaBr₂, from greater than zero to about 10 weight percent CaCl₂, and fromabout 31 to about 40 weight percent water, said modifier being presentin an amount of from about 2 to about 5 weight percent.
 13. Thecomposition of claim 11, wherein said mixture comprises from about 52 toabout 65 weight percent CaBr₂, from greater than zero to about 10 weightpercent CaCl₂, and from about 33 to about 36 weight percent water, saidmodifier being present in an amount of from about 3 to about 4 weightpercent.
 14. The composition of claim 1, including one or morenucleating agents in the composition in an amount sufficient to reducesupercooling of the composition to less than 3° C. during retrieval ofthe stored energy by crystallization.
 15. The composition of claim 14,wherein the nucleating agent is added in an amount of from greater thanzero to about 5.0 weight percent.
 16. The composition of claim 14,wherein the nucleating agent is selected from the group consisting ofSrCl₂, Ba(OH)₂, SrBr₂, Sr(OH)₂, SrI₂, BaI₂, BaO, BaCO₃, SrCO₃ andmixtures thereof.
 17. A reversible liquid/solid phase change compositionhaving a melting temperature of from about 5° C. to about 50° C., saidcomposition comprising a hydrated mixture of from about 20 to about 67weight percent CaBr₂, from greater than zero to about 38 weight percentCaCl₂, from about 28 weight percent water up to about 6.0 moles of waterper mole of calcium salt and from greater than 6.14 moles of water permole of calcium salt up to about 50 weight percent water and fromgreater than zero to less than about 10 weight pecent KCl but sufficientto modify the semicongruent melting behavior of the CaBr₂ /CaCl₂ mixtureto reduce, during freezing of the composition, the formation ofcrystalline CaBr₂ and CaCl₂ hydrate phases other than the hexahydratephase.
 18. The composition of claim 17, wherein said mixture comprisesfrom about 28 to about 43 weight percent CaBr₂, from about 14 to about31 weight percent CaCl₂.
 19. The composition of claim 18, wherein saidmixture comprises from about 30 to about 41 weight percent CaBr₂, fromabout 18 to about 26 weight percent CaCl₂, and from about 2 to about 5weight percent of the modifier.
 20. The composition of claim 18 whereinsaid admixture comprises from about 32 to about 37 weight percent CaBr₂,from about 20 to about 24 weight percent CaCl₂, and from about 3 toabout 4 weight percent of the modifier.
 21. The composition of claim 17,wherein said mixture comprises from about 38 to about 55 weight percentCaBr₂, from about 10 to about 25 weight percent CaCl₂.
 22. Thecomposition of claim 21, wherein said mixture comprises from about 42 toabout 55 weight percent CaBr₂, from about 10 to about 17 weight percentCaCl₂, and said modifier being present in an amount of from about 2 toabout 5 weight percent.
 23. The composition of claim 21, wherein saidmixture comprises from about 46 to about 52 weight percent CaBr₂, fromabout 10 to about 15 weight percent CaCl₂, and said modifier beingpresent in an amount of from about 3 to about 4 weight percent.
 24. Thecomposition of claim 17, wherein said mixture comprises from about 20 toabout 28 weight percent CaBr₂, and from about 21 to about 38 weightpercent CaCl₂.
 25. The composition of claim 24, wherein said mixturecomprises from about 20 to about 28 weight percent CaBr₂, from about 25to about 36 weight percent CaCl₂, and said modifier being present in anamount of from about 2 to about 5 weight percent.
 26. The composition ofclaim 24, wherein said mixture comprises from about 20 to about 28weight percent CaBr₂, from about 28 to about 35 weight percent CaCl₂,and said modifier being present in an amount of from about 3 to about 4weight percent.
 27. The composition of claim 17, wherein said mixturecomprises from about 47 to about 67 weight percent CaBr₂, and fromgreater than zero to about 10 weight percent CaCl₂.
 28. The compositionof claim 27, wherein said mixture comprises from about 50 to about 66weight percent CaBr₂, from greater than zero to about 10 weight percentCaCl₂, and said modifier being present in an amount of from about 2 toabout 5 weight percent.
 29. The composition of claim 27, wherein saidmixture comprises from about 52 to about 65 weight percent CaBr₂, fromgreater than zero to about 10 weight percent CaCl₂, and said modifierbeing present in an amount of from about 3 to about 4 weight percent.30. The composition of claim 17, including one or more nucleating agentsin an amount sufficient to reduce supercooling of the composition toless than 3° C. during retrieval of the stored energy bycrystallization.
 31. The composition of claim 30, wherein the nucleatingagent is present in an amount of from greater than zero to about 5.0weight percent.
 32. The composition of claim 30, wherein the nucleatingagent is selected from the group consisting of SrCl₂, Ba(OH)₂, SrBr₂,Sr(OH)₂, SrI₂, BaI₂, BaO, BaCO₃, SrCO₃, and mixtures thereof.
 33. Anenergy storage device comprising an encapsulating means containing thereversible liquid/solid phase change composition of claim 1 or 17,wherein said phase change composition is hermetically sealed in saidencapsulating means to prevent the evaporation of water from the phasechange composition.
 34. A method of storing energy, comprising the stepsof preparing a reversible liquid/solid phase change composition whichmelts at a temperature of from about 5° to about 50° C., by admixingfrom about 20 to about 67 weight percent CaBr₂, from greater than zeroto about 38 weight percent CaCl₂, from about 28 to about 50 weightpercent water, and a modifier in an amount greater than zero but lessthan about 10 weight percent and in an amount sufficient to modify thesemicongruent melting behavior of the CaBr/CaCl₂ mixture to reduce,during freezing of the composition, the formation of crystalline hydratephases other than the hexahydrate phase, said modifier being selectedfrom the group consisting of KBr, and mixtures of KBr and KCl,introducing the composition into an encapsulating means for use as anenergy storage device, and hermetically sealing the encapsulating meansto prevent the escape of water vapors from the encapsulating means. 35.The method of claim 34, including the step of adding a nucleating agentin an amount sufficient to reduce supercooling of the composition toless than 3° C. during retrieval of the stored energy bycrystallization, said nucleating agent being selected from the groupconsisting of SrCl₂, Ba(OH)₂, BaO, SrCo₃, SrBr₂, Sr(OH)₂, SrI₂, BaI,BaCO₃, and mixtures thereof, said nucleating agent being added in anamount of from greater than zero to about 5.0 weight percent.
 36. Amethod of storing energy, comprising the steps of preparing a reversibleliquid/solid phase change composition which melts at a temperature offrom about 5° to about 50° C., by admixing from about 20 to about 67weight percent CaBr₂, from greater than zero to about 38 weight percentCaCl₂, and from about 28 weight percent water up to about 6.0 moles ofwater per mole of calcium salt and an amount greater than 6.14 moles ofwater per mole of calcium salt up to about 50 weight percent water, andKCl as a modifier in an amount greater than zero but less than about 10weight percent but in an amount sufficient to modify the semicongruentmelting behavior of the CaBr/CaCl₂ mixture to reduce, during freezing ofthe composition, the formation of crystalline hydrate phases other thanthe hexahydrate phase, introducing the composition into an encapsulatingmeans for use as an energy storage device, and hermetically sealing theencapsulating means to prevent the escape of water vapors from theencapsulating means.
 37. The method of claim 36, including the step ofadding a nucleating agent in an amount sufficient to reduce supercoolingof the composition to less than 3° C. during retrieval of the storedenergy by crystallization, said nucleating agent being selected from thegroup consisting of SrCl₂, Ba(OH)₂, BaO, SrBr₂, Sr(OH)₂, SrCO₃, SrI₂,BaI, BaCO₃, and mixtures thereof, said nucleating agent being added inan amount of from greater than zero to about 5.0 weight percent.